The use of image intensifiers has found many usages, including observations at low light levels, e.g. at night, or to create a visible image from electromagnetic radiation invisible to the human eye. An image intensifier tube is generally comprised of a sealed cylindrical enevelope having its one end closed by an entry window on the inner surface of which a photo-sensitive, electron emitting layer in electrical contact with, and enclosed by, a substantially circular, electrically conductive rim. The outer end of the cylindrical envelope is closed by an exit window on which an anode in the form of a phosphor screen is deposited. Means is provided for focussing a beam of electrons released from the photosensitive, electron emitting layer by incident radiation and comprising at least a cathode flange mounted around the substantially circular electrically conductive rim. The cathode flange is sealed by fritting to the entry window and is electrically connected to an electrically conductive, cylindrical member constituting part of the envelope. The image intensifier further comprised a source of voltage located outside the envelope having its positive terminal connected to the anode to supply voltages to the anode and a focussing means. An electric field formed within the enevelope focusses the beam of electrons released from the photosensitive, electron emitting layer onto the anode of the image intensifier device.